clang::Cursor::enum_value_signed should return Option<i64>

[fitzgen]

Right now it always returns i64, but the FFI call it makes says

http://clang.llvm.org/doxygen/group__CINDEX__TYPES.html#ga6b8585818420e7512feb4c9d209b4f4d

Retrieve the integer value of an enum constant declaration as a signed long long.

If the cursor does not reference an enum constant declaration, LLONG_MIN is returned. Since this is also potentially a valid constant value, the kind of the cursor must be verified before calling this function.

We shouldn't force callers to check the kind of the cursor, we should do the check ourselves in this function and return an option. This would result in one less foot gun.

I can mentor anyone who'd like to pick up this bug.

[highfive]

Please make a comment here if you intend to work on this issue. Thank you!

[jcdyer]

I'll take on this one.

[jcdyer]

I'm assuming enum_value_signed is the same thing as enum_val_signed.

[jcdyer]

I took a stab at reorganizing the caller to make more fluent use of the Option, which should make things more straightforward when #128 lands.